Polyaspartic acids and salts thereof are known as an environmentally-adapted biodegradable water-soluble polymer, and are expected to be an alternative as a chelating agent, a scale inhibitor, a detergent builder, and a dispersing agent, and a fertilizer additive for industrial use. Also, for agriculture use, it is known that they have an advantageous effect as a crop growth promoter by mixing it to a fertilizer, as well as a value as an insecticide or a sterilizer is also recognized.
As a method for producing a polyaspartic acid and a salt thereof, non-patent document 1 discloses a method in which aspartic acid is heat condensed at 200° C. for 2 to 3 hours to obtain a polysuccinimide and this is hydrolyzed to produce a polyaspartic acid with a molecular weight of 10000. Patent document 1 discloses a method in which aspartic acid is polymerized under nitrogen atmosphere at a temperature of 180° C. or more for 3 to 6 hours with stirring in a fluidized bed to obtain a polysuccinimide and this is hydrolyzed to produce a polyaspartic acid. Patent document 2 discloses a method in which aspartic acid is heated to 200 to 230° C. in a water-insoluble solvent to obtain a polysuccinimide and this is hydrolyzed by an alkaline aqueous solution to produce a polyaspartic acid salt. In the description of Example 1, a polyaspartic acid salt with a weight average molecular weight (Mw) of 24000 is obtained.
By each above-mentioned method, a polyaspartic acid and a salt thereof with an Mw of 10000 or more can relatively easily be produced. However, since there is a problem that the polysuccinimide is not melted but is consolidated, the process comes to be cumbersome such that a specific apparatus is used to crush it and a high boiling point solvent is used to conduct suspension polymerization. Also, it is disadvantageous as an industrial process that aspartic acid which is a raw material is expensive.
On the other hand, there is a method in which maleamic acid or maleic anhydride and ammonia is used as a raw material to obtain a polysuccinimide and this is hydrolyzed to produce a polyaspartic acid. For example, patent document 3 discloses a method in which maleamic acid is heated to a temperature of 160 to 330° C. to obtain a polysuccinimide. Also, patent document 4 discloses a method in which maleic anhydride is reacted with an aqueous ammonia in water solvent and is then heated to a temperature of at least 170° C. to obtain a polysuccinimide and this is hydrolyzed by a base to produce a polyaspartic acid salt.
Each above-mentioned method is industrially an advantageous method because relatively inexpensive maleic anhydride and ammonia are used as a raw material. However, it involves a problem regarding consolidation of the polysuccinimide as in the case of polyaspartic acid raw material. In order to solve the problem, for example, the patent document 3 proposes (1) using a processing aid such as zeolite or silicate, (2) using a diluent such as tetrahydronaphthalene or a surfactant, and (3) using a solvent such as sulfolane or dimethylsulfoxide. However, in the methods (1) and (2), the process is cumbersome because the processing aid and the diluent must be removed. Also, in the method (3), the process is also cumbersome because the polysuccinimide must be isolated by a method such as reprecipitation. Furthermore, the Mw of the polyaspartic acid described in the Example is approximately 2000. Further, since the polymerization temperature high, it comes to be a problem that coloration of the polymer is large.
Also, in patent document 5, aspartic acid or product obtained from maleic anhydride and ammonia is heat polymerized in water solvent, and is hydrolyzed to produce a polyaspartic acid salt. However, since the polymerization temperature must be 150 to 300° C., it becomes a high pressure reaction and a reactor for high pressure reaction must be used. Further, as for the molecular weight of the polyaspartic acid obtained, it is described that Mw=500 to 10000 (preferably Mw=1000 to 5000), and it is difficult to obtain a polyaspartic acid with an Mw of more than 10000. Also, since the polymerization temperature high, it comes to be a problem that coloration of the polymer is large.
Also, patent document 6, patent document 7, and patent document 8 disclose a method in which maleamic acid or a product obtained from maleic anhydride and ammonia is polymerized in a continuous polymerization apparatus and the polysuccinimide obtained is hydrolyzed to produce a polyaspartic acid salt. However, in these methods, a specific polymerization apparatus is needed overcome difficulty of bulk polymerization. Also, as for the molecular weight of the polyaspartic acid and the salt thereof obtained, it is described that Mw=500 to 10000 (preferably Mw=1000 to 5000) due to the limitation of the retention time, and it is difficult to obtain a polyaspartic acid with an Mw of more than 10000.
On the other hand, patent document 9 discloses a method for producing a polyaspartic acid in which maleamic acid or a product obtained from maleic anhydride and ammonia is hydrogen transfer polymerized in a presence of a vinyl polymerization inhibitor and a basic catalyst. In the description of Example, it is described that maleamic acid is polymerized in a presence of hydroquinone in tetrahydrofuran solvent using sodium t-butoxide or sodium hydroxide as a catalyst to obtain a polyaspartic acid with Mw>20000. However, although there is no detailed description in the Example, since there is a description that water is generated as a by-product during the reaction, a polysuccinimide is produced also in this reaction and the must be cumbersome process is unavoidable such that it must be isolated by reprecipitation.
As is clear from the problem of each above-mentioned prior art, there hardly seems to be a satisfactory method in which a polyaspartic acid and a salt thereof are industrially easily produced from maleamic acid or a product obtained from maleic anhydride and ammonia that is inexpensive in the prior art.